Composite filter media

ABSTRACT

A composite filter media structure includes, in an exemplary embodiment, a base substrate that includes a nonwoven synthetic fabric formed from a plurality of fibers with a spunbond process. The base substrate has a filtration efficiency of about 35% to less than 50%, measured in accordance with EN 1822 (1998) test procedure A nanofiber layer is deposited on one side of the base substrate. The composite filter media structure has a minimum filtration efficiency of about 70%, measured in accordance with EN 1822 (1998) test procedure.

BACKGROUND OF THE INVENTION

The field of the invention relates generally to a composite nonwovenfilter media, and more particularly, to a spunbond nonwoven filter mediahaving a nanofiber based layer applied to at least one surface.

Some known filter media composite constructs incorporate a wet-laidpaper making process to produce the substrate, and an electro-spuntechnology to deposit a lightweight nanofiber coating on one or bothsides of the filter media substrate. Typically the media substrate has abasis weight of 100-120 grams per square meter (g/m²), and the nanofiberlayer has a basis weight of 0.1 g/m² or less.

It is known that the lightweight nanofiber layer is vulnerable to damagein high mechanical stress applications, especially because the nanofiberlayer is formed from fibers with diameters less than 500 nanometer (nm),and more typically, 100 nm. It is known that there are “shedding”problems where the nanofibers are shed from the filter media because ofrelatively weak attraction bonds between the nanofibers and the basemedia for conventional electro-spun fibers that rely on polarityattraction forces. Also, known electro-spun nanofiber layers are twodimensional in structure or a single fiber layer in thickness, and whenthe nanofiber layer cracks or breaks, dust can readily penetrate thebase media substrate After the nanofiber layer is damaged, dust ispermitted to penetrate the base media and contribute to a rise in theoperating pressure drop of the filter. Further, known media substratesalso have mechanical stress limitations and are prone to deformationunder high dust loading.

These known filter media composite constructs when used to filter inletair of power generation gas turbines can permit fine dust particulatesto penetrate the filter over the operating life of the filter.Typically, this known filter media type will have a new or cleanoperating electrically neutral efficiency providing for around 55% ofcapture of 0.4 μm particles, at a pressure drop typically greater than7.0 mm H₂O, and a Quality Factor less than 300, when tested inaccordance with the EN 1822 (1998) test procedure at the known operatingflow rate. It is known that as much as 15 to 20 pounds of dust canpenetrate known filter media over a 24,000 hour operating life becauseof this low initial efficiency. Exposing the turbine blades to dust overan extended time can cause serious and catastrophic fouling and erosionof the turbine blades. The current procedure of cleaning the turbineblades requires taking the turbine off-line at periodic intervals towater wash the blades clean. Turbine down time is expensive because theturbine is not operating and therefore, power generation is curtailed.It would be desirable to provide a higher efficiency filter media thanthe known filter media at a similar or reduced pressure drop to reduceor eliminate turbine down time to clean the turbine blades and/or thereplacement of damaged blades.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a composite filter media structure is provided. Thecomposite filter media structure includes a base substrate that includesa nonwoven synthetic fabric formed from a plurality of fibers with aspunbond process. The base substrate has a filtration efficiency ofabout 35% to less than 50%, measured in accordance with EN 1822 (1998)test procedure. A nanofiber layer is deposited on one side of the basesubstrate. The composite filter media structure has a minimum filtrationefficiency of about 70%, measured in accordance with EN 1822 (1998) testprocedure.

In another aspect, a filter element is provided. The filter elementincludes a first end cap, a second end cap, and a composite filter mediastructure. The composite filter media structure includes a basesubstrate that includes a nonwoven synthetic fabric formed from aplurality of fibers with a spunbond process, the base substrate having afiltration efficiency of about 35% to less than 50%, measured inaccordance with EN 1822 (1998) test procedure. A nanofiber layer isdeposited on one side of the base substrate. The composite filter mediastructure has a minimum filtration efficiency of about 70%, measured inaccordance with EN 1822 (1998) test procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is cross sectional illustration of an exemplary embodiment of acomposite filter media.

FIG. 2 is a photomicrograph of the fibers shown in FIG. 1.

FIG. 3 is a photomicrograph of the base media substrate shown in FIG. 1.

FIG. 4 is a top illustration of the bond pattern of the base mediasubstrate shown in FIG. 1

FIG. 5 is a side illustration of a filter cartridge that includes thefilter media shown in FIG. 1.

FIG. 6 is a perspective illustration of a filter assembly that includesthe filter cartridge shown in FIG. 4.

FIG. 7 is a bar graph of fractional efficiency at 0.3 microns of basemedia substrates at various basis weights in accordance with anexemplary embodiment.

FIG. 8 is a bar graph of fractional efficiency at 0.3 microns of basemedia substrates with and without a nanofiber layer in accordance withan exemplary embodiment compared to a comparative base media substratewith and without a nanofiber layer.

FIG. 9 is a bar graph of pressure drop versus base media substrate withand without a nanofiber layer in accordance with an exemplary aspectcompared to a comparative base media substrate with and without ananofiber layer.

DETAILED DESCRIPTION OF THE INVENTION

A composite filter media for filter assemblies is described in detailbelow. In an exemplary embodiment, the composite filter media includes amedia substrate of a synthetic nonwoven fabric that is formed from twolayers of fibers by a unique spunbond process. A nanofiber layer isdeposited on at least one side of the media substrate. The compositemedia provides an initial filtration efficiency of about 70% or greaterretained capture of 0.4 μm particles when tested in accordance with theEuropean Standard EN 1822 (1998) test procedure, which is about a 15%increase in performance compared to known filter media. In addition, thecomposite media provides the 70% efficiency at a greater than 30% lowerpressure drop than known filter media. The base substrate has afiltration efficiency between about 35% to less than 50% measured inaccordance with EN 1822 (1998) test procedure.

The composite filter media is more durable than known filter media andprovides for lower pressure drop build-up because of less deflection ofthe filter media from the forces exerted on the filter media during thefiltering and reverse cleaning operations. The composite filter mediamay have a quality factor (Q_(f)) of greater than about 370, and in thepreferred embodiment, greater than about 440. Also, the composite filtermedia may have a resistance (or pressure drop) of less than 4.0 mmwater, measured in accordance with EN-1822 (1998), with the base mediasubstrate having a resistance of less than about 2.5 mm water, measuredin accordance with EN-1822 (1998). Further, the nanofiber membrane layerhas a higher basis weight than known filter media which permits thefilter media to clean down more effectively under reverse pulse cleaningthan known filter media. The high basis weight of the nanofiber layerprovides for a durable three dimensional surface filtration layer whichhas an extensive tortuous path that permits high efficiency and fineparticle capture without substantially restricting air flow orincreasing pressure drop.

By “quality factor (Q_(f))” is meant the parameter defined by theequation:Q _(f)=−25000·log(P/100)/ΔpWhere “P”=particle penetration in % and “Δp”=pressure drop across themedia in Pascals.

By “resistance” is meant the resistance (pressure drop) as measuredusing the test method described in EN 1822 (1998).

Referring to the drawings, FIG. 1 is a sectional illustration of anexemplary embodiment of a filter media 10. Filter media 10 includes abase media substrate 12 having a first side 14 and a second side 16. Ananofiber layer 20 is deposited onto first side 14 of media substrate.In another embodiment, nanofiber layer 20 is deposited onto second side16, and in another embodiment, nanofiber layer 20 is deposited on eachof first and second sides 14 and 16.

Media substrate 12 is a nonwoven fabric formed from synthetic fibersusing a spunbond process. The nonwoven fabric comprises dual fibercross-section shapes. Suitable dual fiber layer cross-sections can havefiber shapes having a round structure, or a trilobal structure.Referring also to FIG. 2, in the exemplary embodiment, a dual fibercross-section 30 includes a layer of cylindrical shaped fibers 32 and alayer of trilobal shaped fibers 33. Fibers 32 and 33 are meltspunthrough jets into a plurality of continuous fibers which are uniformlydeposited into a random three dimensional web. The web is then heatedand embossed calendered which thermally bonds the web into aconsolidated spunbond fabric 36, shown in FIG. 3. Heat from contact ofthe calender roll embossing pattern softens or melts the thermoplasticfibers 30 which binds the nonwoven fibers together at the contact pointsof calender roll embossing pattern. The temperature is selected so thatat least softening or fusing of the fibers 30 occurs. In one embodiment,the temperature is about 90° C. to about 240° C. The desired connectionof the fibers is caused by the melting and re-solidification of thefibers 32 and 33 after cooling.

Round fibers 32 have diameter of about 18 microns to about 23 micronsand trilobal fibers 33 have point to point cross-section distances ofabout 22-30 microns.

Referring to FIG. 4, a bond pattern 40 on base media 12 attains anacceptable durability to base media 12, while allowing more fiber to beavailable for filtration thus increasing filtration efficiency. Bondpattern 40 includes a plurality of parallel discontinuous lines 42 ofbond area extending across base media 12. The parallel discontinuouslines 42 of bond area are off-set from each other so that at a locationof no bond area 44 in a discontinuous line 42 is aligned with a bondarea 46 of an adjacent discontinuous line 42. In the exemplaryembodiment the basis weight of base media 12 is about 100 g/m² to about330 g/m, in another embodiment, about 100 g/m² to about 220 g/m².

Any suitable synthetic fiber can be used to make the nonwoven fabric ofmedia substrate 12. Suitable materials for round 32 and trilobal 33fibers include, but are not limited to, polyester, polyamide,polyolefin, thermoplastic polyurethane, polyetherimide, polyphenylether, polyphenylene sulfide, polysulfone, aramid, and mixtures thereof.

In the exemplary embodiment, nanofiber layer 20 is formed by anelectro-blown spinning process that includes feeding a polymer solutioninto a spinning nozzle, applying a high voltage to the spinning nozzle,and discharging the polymer solution through the spinning nozzle whileinjecting compressed air into the lower end of the spinning nozzle. Theapplied high voltage ranges from about 1 kV to about 300 kV. Theelectro-blown spinning process of forming nanofibers and the uniqueapparatus used is described in detail in U.S. Patent ApplicationPublication No. 2005/00677332. The electro-blown spinning processprovides a durable three dimensional filtration layer of nanofibers thatis thicker than known nanofiber filtration layers on known filter media.In the exemplary embodiment the basis weight of nanofiber layer 20 isabout 0.6 g/m² to about 20 g/m², in another embodiment, about 2 g/m² toabout 20 g/m², in another embodiment, about 5 g/m² to about 10 g/m², inanother embodiment, about 1.5 g/m² to about 2.5 g/m². The nanofibers innanofiber layer 20 have an average diameter of about 500 nm or less.

In alternate embodiments, nanofiber layer 20 may be formed byelectrospinning, centrifugal spinning, or melt blowing. Classicalelectrospinning is a technique described in detail in U.S. Pat. No.4,127,706. A high voltage is applied to a polymer in solution to createnanofibers and nonwoven mats. However, total throughput inelectrospinning processes is too low to be viable in forming heavierbasis weight webs. Centrifugal spinning is a fiber forming process thatincludes supplying a spinning solution having at least one polymerdissolved in at least one solvent to a rotary sprayer having a rotatingconical nozzle. The nozzle has a concave inner surface and a forwardsurface discharge edge. The spinning solution moves through the rotarysprayer along the concave inner surface so as to distribute the spinningsolution toward the forward surface of the discharge edge of the nozzle.Separate fibrous streams are formed from the spinning solution while thesolvent vaporizes to produce polymeric fibers in the presence or absenceof an electrical field. A shaping fluid can flow around the nozzle todirect the spinning solution away from the rotary sprayer. The fibersare collected onto a collector to form a nanofiber web. In addition,melt blowing is described in detail in U.S. Pat. No. 6,520,425.

Suitable polymers for forming nanofibers by the electro-blown spinningprocess are not restricted to thermoplastic polymers, and may includethermosetting polymers. Suitable polymers include, but are not limitedto, polyimides, polyamides (nylon), polyaramides, polybenzimidazoles,polyetherimides, polyacrylonitriles, polyethylene terephthalate,polypropylene, polyanilines, polyethylene oxides, polyethylenenaphthalates, polybutylene terephthalate, styrene butadiene rubber,polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidenechloride, polyvinyl butylene, polyacetal, polyamide, polyester,polyolefins, cellulose ether and ester, polyalkylene sulfide,polyarylene oxide, polysulfone, modified polysulfone polymers, andmixtures thereof. Also, materials that fall within the generic classesof poly (vinylchloride), polymethylmethacrylate (and other acrylicresins), polystyrene, and copolymers thereof (including ABA type blockcopolymers), poly (vinylidene fluoride), poly (vinylidene chloride),polyvinylalcohol in various degrees of hydrolysis (87% to 99.5%) incrosslinked and non-crosslinked forms may be used and copolymer orderivative compounds thereof. One suitable class of polyamidecondensation polymers are nylon materials, such as nylon-6, nylon-6, 6,nylon 6, 6-6, 10, and the like. The polymer solution is prepared byselecting a solvent that dissolves the selected polymers. The polymersolution can be mixed with additives, for example, plasticizers,ultraviolet ray stabilizers, crosslink agents, curing agents, reactioninitiators, and the like. Although dissolving the polymers may notrequire any specific temperature ranges, heating may be needed forassisting the dissolution reaction.

It can be advantageous to add plasticizers to the various polymersdescribed above, in order to reduce the T_(g) of the fiber polymer.Suitable plasticizers will depend upon the polymer, as well as upon theparticular end use of the nanofiber layer. For example, nylon polymerscan be plasticized with water or even residual solvent remaining fromthe electrospinning or electro-blown spinning process. Otherplasticizers which can be useful in lowering polymer T_(g) include, butare not limited to, aliphatic glycols, aromatic sulphanomides, phthalateesters, including but not limited to, dibutyl phthalate, dihexylphthalate, dicyclohexyl phthalate, dioctyl phthalate, diisodecylphthalate, diundecyl phthalate, didodecanyl phthalate, and diphenylphthalate, and the like.

FIG. 5 is a side illustration of a filter element 50 formed from filtermedia 10. In the exemplary embodiment, filter media 10 includes aplurality of pleats 52. Filter element 50 includes a first end cap 54and an opposing second end cap 56 with filter media 10 extending betweenend caps 54 and 56. Filter element 50 has a tubular shape with aninterior conduit 58 (shown in FIG. 6). Filter element 50 is cylindricalin shape, but can also be conical as shown in FIG. 6. Filter element 50can also include an inner and/or an outer support liner to providestructural integrity of filter element 50 and/or support for filtermedia 10.

FIG. 6 is a perspective illustration of a filter assembly 60 thatincludes a plurality of filter elements 50 mounted to a tube sheet 62 inpairs in an end to end relationship. Tube sheet 62 separates the dirtyair side from the clean air side of filter assembly 60. A cleaningsystem 64 for cleaning filter elements 50 with pulsed air includes aplurality of air nozzles 66 mounted to air supply pipes 68. Pulses ofcompressed air directed into interior conduit 58 of filter elements 50are used to clean filter elements 50 of collected dirt and dust.

Flat sheets of base media substrate 12 test samples having various basisweights were compared to a comparative base media substrate in a flatsheet fractional efficiency test in accordance with EN 1822 (1998) testmethod. Air containing DEHS particles was directed through each testsample at a flow rate of about 5.3 cm/s. FIG. 7 shows a graphicalrepresentation of the comparison test and the enhanced filtrationefficiency performance of spunbond base media 12. Bar A represents basesubstrate 12 at a basis weight of 165 g/m², and Bar B represents acomparative base substrate at a basis weight of 230 g/m². Bar Crepresents a comparative base media substrate with a basis weight of 130g/m². The base media substrates did not include a nanofiber layer. Basemedia substrate 12 has a higher efficiency than the comparative base atthe 0.3 micron particle size tested at 5.3 cm/s

Flat sheets of base media substrate 12, and base media substrate 12including nanofiber layer 20 were compared to a comparative base mediasubstrate with and without a nanofiber layer in a flat sheet fractionalefficiency test in accordance with EN 1822 (1998) test method. Aircontaining 0.3 micron DEHS particles was directed through each testsample at a flow rate of about 5.3 cm/s. FIG. 8 shows a graphicalrepresentation of the comparison test. Bar A represents base mediasubstrate 12 at 165 g/m², and Bar B represents base media substrate 12at 165 g/m², including nanofiber layer 20. Bar C represents acomparative base media substrate and Bar D represents the comparativebase media substrate including a nanofiber layer. Base media substrate12 with and without nanofiber layer 20 had a higher efficiency than thecomparative base substrate with and without a nanofiber layer.

Flat sheets of base media substrate 12, and base media substrate 12including nanofiber layer 20 were compared to a comparative base mediasubstrate with and without a nanofiber layer in a flat sheet pressuredrop test in accordance with EN 1822 (1998) test method. Air containingDEHS particles was directed through each test sample at a flow rate ofabout 5.3 cm/s FIG. 9 shows a graphical representation of the comparisontest. Bar A represents a comparative base media substrate and bar Brepresents the comparative base media substrate including a nanofiberlayer. Bar C represents base media substrate 12 at 165 g/m², and bar Drepresents base media substrate 12 at 165 g/m², including nanofiberlayer 20. Base media substrate 12 with and without nanofiber layer 20had a lower pressure drop than the comparative base substrate with andwithout a nanofiber layer.

The above described filter elements 50 formed from filter media 10 canbe used for filtering an air stream in almost any application, forexample, for filtering gas turbine inlet air. The unique construction offilter media 10 is more durable than known filter media and provides forrelatively lower pressure drop build-up because of less deflection fromthe forces exerted on the filter media during the filtering and reversecleaning operations. Filter elements 50 can produce an averageefficiency greater than about 70% capture of the most penetratingparticle size of aerosol or dust (about 0.3 to about 0.4 micron) ascompared to an efficiency of about 50-55% of known filter elements.Also, nanofiber layer 20 has a higher basis weight than known filtermedia which permits filter media 10 to clean down more effectively underreverse pulse cleaning than known filter media. Further, the high basisweight of nanofiber layer 20 provides for a durable three dimensionalsurface filtration layer which has an extensive tortuous path thatpermits high efficiency and fine particle capture without restrictingair flow or increasing pressure drop.

The example filter media of Examples 1 and 2 and Comparative Examples3-7 illustrate a comparison of embodiments of filter media 10 with knownfilter media. Efficiency, resistance and quality factor were measuredfor each filter media of Examples 1 and 2 and Comparative Examples 3-7.Efficiency was measured in accordance with EN-1822 (1998) testprocedure, resistance was measured in accordance with EN-1822 (1998),and quality factor Q_(f) was calculated as described above.

Example 1 is a spunbond polyester dual layer base media substratecontaining round and trilobal fibers, and Example 2 is the base mediasubstrate of Example 1 plus a 2 g/m² nanofiber layer formed by aelectro-blown spinning process. Comparative Example 3 is a knowndry-laid polyester base media substrate, and Comparative Example 4 isthe known dry-laid polyester base media substrate of Comparative Example3 plus a 2 g/m² nanofiber layer. Comparative Example 5 is a wet-laidsynthetic paper plus a <0.5 g/m² nanofiber layer. Comparative Example 6is a wet-laid synthetic paper, and Comparative Example 7 is the wet-laidsynthetic paper of Example 6 plus a 20 g/m² meltblown fiber layer. Theexample results are shown in Table I below. When Example 2 is comparedto composites in Comparative Examples 4, 5, and 7, efficiency is notsacrificed at the expense of reducing resistance which yields theassociated high Quality Factor values.

TABLE I Basis Weight Efficiency Resistance Quality Example (g/m²) (%)(mm H₂O) Factor Example 1 169.9 39.4 2.07 267 Spunbond Polyester DualLayer Fiber Base Example 2 170.3 71.4 3.1 447 Spunbond Polyester DualLayer Fiber Base + 2 g/m² Nanofiber Layer Comparative Example 3 234.928.7 9.3 40 Drylaid Polyester Base Comparative Example 4 236.3 43.213.81 45 Drylaid Polyester Base + 2 g/m² Nanofiber Layer ComparativeExample 5 121.2 40.5 9.77 59 Wet laid Synthetic Paper + <0.5 g/m²Nanofiber Layer Comparative Example 6 133.4 9.0 7.67 14 WetlaidSynthetic Paper Comparative Example 7 150.2 86.4 8.79 251 WetlaidSynthetic Paper + 20 g/m² Meltblown Fiber Layer Efficiency measured at0.3 microns, 5.3 cm/s face velocity EN 1822 (1998). Resistance measuredin accordance with EN-1822 (1998). Quality Factor defined by theequation: Q_(f) = −25000 · log(P/100)/Δp

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. A composite filter media structure comprising: abase substrate comprising a nonwoven synthetic fabric formed from aplurality of fibers with a spunbond process, said base substrate havinga filtration efficiency of about 35% to less than 50% capture ofparticles having a size of 0.3 μm, measured in accordance with EN 1822(1998) test procedure, wherein said nonwoven synthetic fabric iscomprised of a dual layer containing both round and trilobal fibercross-sections; and a nanofiber layer deposited on one side of said basesubstrate, said composite filter media structure having a minimumfiltration efficiency of about 70% capture of particles having a size of0.4 μm, said efficiencies of base substrate and composite media beingmeasured in accordance with EN 1822 (1998) test procedure on a flatsubstrate at a face air velocity of 5.3 cm/second and wherein said basesubstrate has a resistance less than about 2.5 mm of water, measured inaccordance with EN-1822 (1998), and said composite filter mediastructure has a resistance less than about 4.0 mm of water, measured inaccordance with EN-1822 (1998).
 2. A composite filter media structure inaccordance with claim 1, wherein said base substrate and said nanofiberlayer in combination are substantially electrically neutral.
 3. Acomposite filter media structure in accordance with claim 1, whereinsaid nanofiber layer is formed by electro-blown spinning,electrospinning, centrifugal spinning, or melt blowing.
 4. A compositefilter media structure in accordance with claim 3, wherein saidnanofiber layer comprises at least one of polyester, polyamide,polyolefin, thermoplastic polyurethane, polyetherimide, polyphenylether, polyphenylene sulfide, polysulfone, and aramid.
 5. A compositefilter media structure in accordance with claim 1, wherein said basesubstrate has a basis weight of about 100 g/m2 to about 300 g/m2.
 6. Acomposite filter media structure in accordance with claim 1, whereinsaid nanofiber layer comprises a plurality of nanofibers having anaverage diameter of about 500 nm or less, said nanofiber layer having abasis weight of about 0.6 g/m2 to about 20 g/m2.
 7. A composite filtermedia structure in accordance with claim 1, wherein said nanofiber layercomprises a plurality of nanofibers having an average diameter of about500 nm or less, said nanofiber layer having a basis weight of about 1.5g/m2 to about 2.5 g/m2.
 8. A composite filter media structure inaccordance with claim 1, wherein said plurality of fibers comprise anaverage diameter of about 18 to about 30 microns.
 9. A composite filtermedia structure in accordance with claim 1, wherein the quality factorQf of said composite filter media structure is greater than about 370.10. A composite filter media structure in accordance with claim 1,wherein the quality factor Qf of said composite filter media structureis greater than about
 440. 11. A composite filter media structure inaccordance with claim 1, wherein said nonwoven synthetic fabriccomprises a bond area pattern comprising a plurality of substantiallyparallel discontinuous lines of bond area.
 12. A filter elementcomprising a first end cap, a second end cap, and a composite filtermedia structure, the composite filter media structure comprising: a basesubstrate comprising a nonwoven synthetic fabric formed from a pluralityof fibers with a spunbond process, said base substrate having afiltration efficiency of about 35% to less than 50% capture of particleshaving a size of 0.3 μm, measured in accordance with EN 1822 (1998) testprocedure, wherein said nonwoven synthetic fabric is comprised of a duallayer containing both round and trilobal fiber cross-sections; and ananofiber layer deposited on one side of said base substrate, saidcomposite filter media structure having a minimum filtration efficiencyof about 70% capture of particles having a size of 0.4 μm, saidefficiencies of base substrate and composite media being measured inaccordance with EN 1822 (1998) test procedure on a flat substrate at aface air velocity of 5.3 cm/second and wherein said base substrate has aresistance less than about 2.5 mm of water, measured in accordance withEN-1822 (1998), and said composite filter media structure has aresistance less than about 4.0 mm of water, measured in accordance withEN-1822 (1998).
 13. A filter element in accordance with claim 12,wherein said base substrate and said nanofiber layer in combination aresubstantially electrically neutral.
 14. A filter element in accordancewith claim 12, wherein said nanofiber layer is formed by electro-blownspinning, electrospinning, centrifugal spinning, or melt blowing.
 15. Afilter element in accordance with claim 14, wherein said nanofiber layercomprises at least one of polyester, polyamide, polyolefin,thermoplastic polyurethane, polyetherimide, polyphenyl ether,polyphenylene sulfide, polysulfone, and aramid.
 16. A filter element inaccordance with claim 12, wherein said base substrate has a basis weightof about 100 g/m2 to about 300 g/m2.
 17. A filter element in accordancewith claim 12, wherein said nanofiber layer comprises a plurality ofnanofibers having an average diameter of about 500 nm or less, saidnanofiber layer having a basis weight of about 0.6 g/m2 to about 20g/m2.
 18. A filter element in accordance with claim 12, wherein saidnanofiber layer comprises a plurality of nanofibers having an averagediameter of about 500 nm or less, said nanofiber layer having a basisweight of about 1.5 g/m2 to about 2.5 g/m2.
 19. A filter element inaccordance with claim 12, wherein said plurality of fibers comprise anaverage diameter of about 18 to about 30 microns.
 20. A filter elementin accordance with claim 12, wherein the quality factor Qf of saidcomposite filter media structure is greater than about
 370. 21. Acomposite filter media structure in accordance with claim 12, whereinthe quality factor Qf of said composite filter media structure isgreater than about
 440. 22. A filter element in accordance with claim12, wherein said composite filter media structure further comprises aplurality of pleats.
 23. A filter element in accordance with claim 12,wherein said nonwoven synthetic fabric comprises a bond area patterncomprising a plurality of substantially parallel discontinuous lines ofbond area.